Quantum decision affects results of measurements taken earlier in time

An entanglement experiment involving four photons appears to play tricks...

Quantum entanglement is a state where two particles have correlated properties: when you make a measurement on one, it constrains the outcome of the measurement on the second, even if the two particles are widely separated. It's also possible to entangle more than two particles, and even to spread out the entanglements over time, so that a system that was only partly entangled at the start is made fully entangled later on.

This sequential process goes under the clunky name of "delayed-choice entanglement swapping." And, as described in a Nature Physics article by Xiao-song Ma et al., it has a rather counterintuitive consequence. You can take a measurement before the final entanglement takes place, but the measurement's results depend on whether or not you subsequently perform the entanglement.

Delayed-choice entanglement swapping consists of the following steps. (I use the same names for the fictional experimenters as in the paper for convenience, but note that they represent acts of measurement, not literal people.)

Two independent sources (labeled I and II) produce pairs photons such that their polarization states are entangled. One photon from I goes to Alice, while one photon from II is sent to Bob. The second photon from each source goes to Victor. (I'm not sure why the third party is named "Victor".)

Alice and Bob independently perform polarization measurements; no communication passes between them during the experiment—they set the orientation of their polarization filters without knowing what the other is doing.

At some time after Alice and Bob perform their measurements, Victor makes a choice (the "delayed choice" in the name). He either allows his two photons from I and II to travel on without doing anything, or he combines them so that their polarization states are entangled. A final measurement determines the polarization state of those two photons.

The results of all four measurements are then compared. If Victor did not entangle his two photons, the photons received by Alice and Bob are uncorrelated with each other: the outcome of their measurements are consistent with random chance. (This is the "entanglement swapping" portion of the name.) If Victor entangled the photons, then Alice and Bob's photons have correlated polarizations—even though they were not part of the same system and never interacted.

The practicalities of delayed-choice entanglement swapping bears many similarities to other entanglement experiments. Ma et al. sent pulsed light from an ultraviolet laser through two separate beta-barium borate (BBO) crystals, which respond by emitting two photons with entangled polarizations, but equal wavelength. The BBO crystals acted as the sources labeled I and II above; the oppositely polarized photons they produced were sent down separate paths. One path for each BBO crystal led to a polarization detector ("Alice" and "Bob"), while the other passed through a fiber-optic cable 104 meters long before arriving at the "Victor" apparatus.

That little bit of cabling was enough to ensure that anything that happened at Victor occurred after Alice and Bob had done their measurements.

The choice about entangling the photons at the Victor apparatus was made by a random-number generator, and passed through a tunable bipartite state analyzer (BiSA). The BiSA contained two beam-splitters that select photons' paths depending on their polarization, along with a device that rotated the polarization of the photons. Depending on the "choice" to entangle or not, the polarization of the photons from I and II were made to correlate or left alone. Finally, the polarization of both photons at Victor were measured, and compared with the results from Alice and Bob.

Due to the 104-meter fiber-optic cable, Victor's measurements occurred at least 14 billionths of a second after those of Alice and Bob, precluding the idea that the setting of the BiSA caused the polarization results to change. While comparatively few photons made it all the way through every step of the experiment, this is due to the difficulty of measurements with so few photons, rather than a problem with the results.

Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.

Suffice it to say that facile explanations about information passing between Alice's and Bob's photons lead to violations of causality, since Alice and Bob perform their polarization measurement before Victor makes his choice about whether to entangle his photons or not. (Similarly, if you think that all the photons come from a single laser source, they must be correlated from the start, and you must answer how they "know" what Victor is going to do before he does it.)

The picture certainly looks like future events influence the past, a view any right-minded physicist would reject. The authors conclude with some strong statements about the nature of physical reality that I'm not willing to delve into (the nature of physical reality is a bit above my pay grade).

As always with entanglement, it's important to note that no information is passing between Alice, Bob, and Victor: the settings on the detectors and the BiSA are set independently, and there's no way to communicate faster than the speed of light. Nevertheless, this experiment provides a realization of one of the fundamental paradoxes of quantum mechanics: that measurements taken at different points in space and time appear to affect each other, even though there is no mechanism that allows information to travel between them.

88 Reader Comments

You mentioned that very few photons would make it through the entire apparatus. How do we know that Victor's sampling wasn't inherently biasing the results to pairs of entangled photons?

If Alice and Bob were to talk on the phone and discuss who had what polarization state before Victor imposed re-entanglement would that change the outcomes? (theoretically speaking since all this happens in nanoseconds)

When Alice and Bob perform the measurements, can they look at the measurements prior to Victor deciding whether to entangle the other photons? If so, would it be possible for Victor to entangle selectively in a way that violates the correlation?

Matt, it would be extremely interesting to hear "the strong statements about reality." It is not above your pay grade and you are remiss in not communicating them. The $32 to read the original paper is above my curiosity budget. So do the readers a favor.

With its dry tone, scholarly references, and handy graphs, Asimov's work appears indistinguishable from any other reasonably important, definitely boring scientific paper detailing the latest research on some chemical compound. It's only when you actually read the thing that you realize just how ridiculous it all is, as Asimov explains how he tried to "trick" the thiotimoline by removing the water after the compound dissolved but before the water was ever actually added:

Quote:

This, fortunately for the law of Conservation of Mass-Energy, never succeeded since solution never took place unless the water was eventually added. The question is, of course, instantly raised as to how the thiotimoline can 'know' in advance whether the water will ultimtaely be added or not. Though this is not properly within our province as physical chemists, much recent material has been published with the last year upon the psychological and philosophical problems thereby posed.

Nevertheless, the chemical difficulties involved rest in the fact that the time of solution varies enormously with the exact mental state of the experimenter. A period of even slight hesitation in adding the water reduces the negative time of solution, not infrequently wiping it out below the limits of detection.

To circumvent this problem, Asimov wrote, it was necessary to create a device that would deal with adding the thiotimoline to the water and accurately measure just how long before this the compound dissolved. This device was called the endochronometer, and indeed Asimov referred to thiotimilone's time-jumping abilities simply as its "endochronic properties." Never at any point is any of this considered strange or worth remarking upon beyond simple description of the experiments, which is part of what makes the gag so effective.

I fully expect that someday, all of what Quantum mechanics describes will be well understood. The results we keep hearing about are not unlike the bizarre paths of stars when we ASSUMED that the earth was at the center of the universe. Once we got that assumption in place, everything worked out better.

My best guess is that our assumption that space and time can be divided infinitely is at the heart of the problem.

I took an introduction to Quantum Mechanics and Relativity in college. So, I know a little but understand nothing.

When Alice and Bob perform the measurements, can they look at the measurements prior to Victor deciding whether to entangle the other photons? If so, would it be possible for Victor to entangle selectively in a way that violates the correlation?

You mentioned that very few photons would make it through the entire apparatus. How do we know that Victor's sampling wasn't inherently biasing the results to pairs of entangled photons?

My thoughts exactly. Given that half of each pair of photons has already undergone measurement, it would seem it might be the case that Victor can only successfully "entangle" the other half of each pair if the already-made (Alice and Bob) measurements are appropriately consistent with the pair being entangled.

When Alice and Bob perform the measurements, can they look at the measurements prior to Victor deciding whether to entangle the other photons? If so, would it be possible for Victor to entangle selectively in a way that violates the correlation?

Perhaps he would die of a heart attack or get stuck by a lightning or something if we wanted to try that.

Wouldn't another way to look at these results be, "The particles knew of their future entanglement before it occurred." In other words, the future event was already set so the present particles acted accordingly. The nice thing about these results is that when looking at the initial states we cannot know if they were the result of random chance or future entanglement.

You mentioned that very few photons would make it through the entire apparatus. How do we know that Victor's sampling wasn't inherently biasing the results to pairs of entangled photons?

My thoughts exactly. Given that half of each pair of photons has already undergone measurement, it would seem it might be the case that Victor can only successfully "entangle" the other half of each pair if the already-made (Alice and Bob) measurements are appropriately consistent with the pair being entangled.

Maybe this would be useful in high speed stock trading. Instead of just butting in line for a stock trade, maybe somebody could find a way to literally place their trade earlier than the person who really wanted to trade. That would be a way to make money without producing any value.

The authors conclude with some strong statements about the nature of physical reality

Part of the section Matt is referring to there is

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According to Wheeler, Bohr said: “No elementary phenomenon is a phenomenon until it is a registered phenomenon.”. We would like to extend this by saying: “Some registered phenomena do not have a meaning unless they are put in relationship with other registered phenomena.”

Isn't that a fancy way of saying 'It ain't over till it's over and it ain't over yet'?

Those who do not understand the lessons of the Einstein-Podolsky-Rosen paradox are doomed to repeat it.

Could you explain more in layman's terms?

EPR posited that quantum entanglement requires information to travel faster than the speed of light, and therefore quantum mechanics has some serious problems.

The Copenhagen interpretation states that the wave function is only a mathematical tool and doesn't describe objective reality, therefore EPR is pretty much irrelevant, because any claimed violations of causality, relativity, etc. are purely mathematical and don't really happen.

I don't see how the result of the experiment in the article is an intuitive result of this interpretation, though.

The paper (http://arxiv.org/ftp/arxiv/papers/1203/1203.4834.pdf) says, "Remarkably, whether the earlier registered results of photons 1 and 4 indicate the existence of entanglement between photons 1 and 4 depends on the later choice of Victor." Could another interpretation be that Victor's choice is constrained by the measurement on photons 1 and 4?

Well this article clearly demonstrates that by measuring time and space we are messing it all up. From now on all clocks and devices/units of measurements are hereby banned to preserve the proper and present reality.

The problem with this interpretation is that the choice in question is statistically random.

Now, the Alice and Bob measurements are probably random too, so what you say may not skew the results. I didn't read the paper, but didn't they use less than 50/50 or more than 50/50 as the chance of entanglement? That would help.

Anyway, some of the big heads here seem to see this result with blasé eyes, so they may have a very good explanation which does not require experimental approximations.

The paper (http://arxiv.org/ftp/arxiv/papers/1203/1203.4834.pdf) says, "Remarkably, whether the earlier registered results of photons 1 and 4 indicate the existence of entanglement between photons 1 and 4 depends on the later choice of Victor." Could another interpretation be that Victor's choice is constrained by the measurement on photons 1 and 4?

Actually, it looks like this possibility is not ruled out by the paper: "Our experiment relies on the assumption of the statistical independence of the QRNG from other events, in particular Alice and Bob’s measurement results. Note that in a conspiratorial fashion, Victor’s choice might not be free but always such that he chooses a separable-state measurement whenever Alice and Bob’s pair is in a separable-state, and he chooses a Bell-state measurement whenever their pair is in an entangled state. This would preserve the viewpoint that in every single run Alice and Bob do receive a particle pair in a definite separable or a definite entangled state. A possible improvement of our set-up would be space-like separation of Victor’s choice event and the measurement events of Alice and Bob to further strength the assumption of the mutual independence of these events."

Matt, it would be extremely interesting to hear "the strong statements about reality." It is not above your pay grade and you are remiss in not communicating them. The $32 to read the original paper is above my curiosity budget. So do the readers a favor.

Pardon my ignorance, but does this effectively mean that information can now be communicated back in time? A message can be made with many units with as few as 2 distinct properties (1 or 0, for example). It sounds to me like the authors were able to communicate one bit of information 14 billionths of a second back in time. Do that 5 times and you can make any letter in the english language (5 bits => 2^5 = 32 possible combinations).

If "victor" could be made to delay for several seconds, and you added a lot more photons to the device, you could compose a message of a few characters. I could imagine an experiment where a computer records the "alice and bob" results and displays them as a textual message on the screen (converting from the binary result set into letters), which a scientist sees. The scientist then tells another scientist to type in a message on another computer, but does not tell the other scientist what the message is. The other computer takes the letters typed in, converts them to a bit sequence, and tells the "victor" unit to entangle on 1's and not entangle on 0's. The other scientist doesn't know what showed up on the other screen prior to typing in the message.

I realize building a machine to keep photons entangled for many seconds is very hard, but haven't some teams managed to do this for longer and longer periods of time already? It seems to me the hardest part was getting the information to go back in time. I also realize that the "noise" - the error rate of reliably reading ("alice and bob") and writing ("victor") is probably very high, but if you add more bits to the message, you can fix the noise with an error correcting code.

Next up, testing the Grandfather Paradox by communicating what shows up on the screen to the other scientist, and the other scientist intentionally entering a different message.

What am I missing? Darn, this makes me want to go back to school and become a physicist as a second career. :-)

Actually, it looks like this possibility is not ruled out by the paper: "Our experiment relies on the assumption of the statistical independence of the QRNG from other events, in particular Alice and Bob’s measurement results. Note that in a conspiratorial fashion, Victor’s choice might not be free but always such that he chooses a separable-state measurement whenever Alice and Bob’s pair is in a separable-state, and he chooses a Bell-state measurement whenever their pair is in an entangled state. This would preserve the viewpoint that in every single run Alice and Bob do receive a particle pair in a definite separable or a definite entangled state. A possible improvement of our set-up would be space-like separation of Victor’s choice event and the measurement events of Alice and Bob to further strength the assumption of the mutual independence of these events."

I'm still trying to wrap my head around this, but one quote from the article below somehow seems fishy.

Quote:

While comparatively few photons made it all the way through every step of the experiment, this is due to the difficulty of measurements with so few photons, rather than a problem with the results.

I wonder if somehow the photons that make it through somehow have their state selected. Is it possible that only the photons that ... argh... /head explodes

I clearly am missing something here. Victor isn't just choosing "entangle" vs "do not entangle", he is also making a measurement. Victor's measurements are then compared to Alice and Bob's, separately for the two entanglement states.

The claim the authors are making (or maybe just Matthew is making the claim?), is that this shows that somehow Victor's choice is affecting Alice and Bob's measurement.

But the interpretation of Alice and Bob's photons as being entangled or not is dependent on Victor's measurements, is it not?

Why is it not the case then, that the photons that enter Victor's apparatus already "know" what Alice and Bob's measurements are, and it is only Victor's results that change when the entanglement state is changed?

The choice about entangling the photons at the Victor apparatus was made by a random-number generator, and passed through a tunable bipartite state analyzer (BiSA).

...

While comparatively few photons made it all the way through every step of the experiment, this is due to the difficulty of measurements with so few photons, rather than a problem with the results.

Ma et al. found to a high degree of confidence that when Victor selected entanglement, Alice and Bob found correlated photon polarizations. This didn't happen when Victor left the photons alone.

First problem I see is "random-number generator". As a computer scientist, I know that such a device is fiction - there does not exist a truly random "random-number generator", only statistical models that approximate such.

Second, the significant numbers of photons which were never measured in the experiment. Why? Would including that data alter the outcome?

And third, the outcome itself. "...a high degree of confidence". Again, this implies more statistics. As in, some of the time, the outcome is as stated, but other times, it is not, Statistically, these failure states are background noise to the true results of the experiment. And yet, when we're talking about implying that somehow the causality principle is being violated, I don't see how you can just throw out data that runs contrary to your experiment.

I clearly am missing something here. Victor isn't just choosing "entangle" vs "do not entangle", he is also making a measurement. Victor's measurements are then compared to Alice and Bob's, separately for the two entanglement states.

The claim the authors are making (or maybe just Matthew is making the claim?), is that this shows that somehow Victor's choice is affecting Alice and Bob's measurement.

But the interpretation of Alice and Bob's photons as being entangled or not is dependent on Victor's measurements, is it not?

Though it's not clear from the Ars article, my understanding is that the measurements that matter are Alice and Bob's. The implication is that if you log XOR(Alice, Bob) you will get random results when Victor is not entangling his half of their pairs and 100% zeros or ones (I'm not sure which) when Victor is entangling the pairs.

The statements "A final measurement determines the polarization state of those two photons." and "The results of all four measurements are then compared." seems to be wrong. But maybe that is the "loose cable" in this experiment?

I clearly am missing something here. Victor isn't just choosing "entangle" vs "do not entangle", he is also making a measurement. Victor's measurements are then compared to Alice and Bob's, separately for the two entanglement states.

The claim the authors are making (or maybe just Matthew is making the claim?), is that this shows that somehow Victor's choice is affecting Alice and Bob's measurement.

But the interpretation of Alice and Bob's photons as being entangled or not is dependent on Victor's measurements, is it not?

Though it's not clear from the Ars article, my understanding is that the measurements that matter are Alice and Bob's. The implication is that if you log XOR(Alice, Bob) you will get random results when Victor is not entangling his half of their pairs and 100% zeros or ones (I'm not sure which) when Victor is entangling the pairs.

The statements "A final measurement determines the polarization state of those two photons." and "The results of all four measurements are then compared." seems to be wrong. But maybe that is the "loose cable" in this experiment?

From the paper (emphasis mine):

Quote:

According to Victor’s choice and his results, Alice and Bob can sort their already recorded data into subsets and can verify that each subset behaves as if it consisted of either entangled or separable pairs of distant photons, which have neither communicated nor interacted in the past.